1. Field of the Invention
The present invention generally relates to an optical spectrometer. More specifically, the present invention relates to a polarization-independent, high resolution, non-scanning optical spectrometer.
2. Technical Background
High resolution spectrometers are used in a wide variety of optical applications particularly in optical communication networks. As just a few examples, spectrometers may be used as a channel monitor in a wavelength division multiplexed (WDM) optical communication network, as a polarization mode dispersion (PMD) sensor, or to measure the optical power spectrum of any optical field propagating through an optical network.
In general, spectrometers are classified as either scanning or non-scanning spectrometers. While scanning spectrometers can exhibit very high spectral resolutions, the fact that they must be scanned limits their usefulness to the domain of optical signals whose spectrum does not change appreciably over the required scan time. Non-scanning spectrometers provide an attractive method for estimating the power spectral density of an incident optical signal because the far-field pattern, which serves as the spectral estimate, is available more or less instantaneously and therefore can be used to estimate the power spectrum of very short-lived signals.
As described further below, spectrometers often utilize diffraction gratings to diffract incident light and form a far-field optical pattern from which a power spectral estimate may be obtained. Diffraction gratings typically include a plurality of periodically and equally spaced grooves that act as scattering sites to scatter the incident light. The light scattered from each of the grooves creates an interference pattern a distance away from the grating (known as the far-field diffraction or Fraunhofer pattern). The resulting far-field pattern exhibits regions or peaks of high intensity for a given wavelength, which are commonly called diffraction orders. Examples of the far-field patterns for two different wavelengths are shown in FIG. 1.
As apparent from FIG. 1, the variation in wavelengths of the incident light signal produces a corresponding shift and stretching out of the diffraction order peaks in the far-field pattern. It is this shifting of the diffraction order peaks that enables the power spectrum to be determined. When a diffraction grating has grooves that are inconsistent in size or not perfectly spaced apart, the far-field pattern changes so that the diffraction order peaks are not well defined. Consequently, such imprecision of the grating makes it more difficult to spatially resolve the far-field pattern and hence makes it more difficult to determine the power spectral density of the incident signal. Accordingly, the resolving power of a spectrometer incorporating a diffraction grating is normally considered to be the degree to which the resonant diffraction order peaks in the far-field can be spatially resolved.
In order to increase the ability to resolve the far-field pattern, the number of grooves in the grating that are illuminated by the incident beam must be significantly increased as must the cross-sectional area of the beam that impinges upon the diffraction grating. This technique is effective until such point that aberrations in the optical system limit the separability of the wavelengths. Such aberrations begin to appear as the spacing between the grooves of the diffraction grating decreases.
To obtain sufficient resolution, the spacing between the grooves of the diffraction grating must be small relative to the wavelength of the incident light signal. In a WDM system, the wavelengths of such light signals are on the order of 1.5 microns. Thus, to obtain sufficient resolution, the spacing between the grooves of the diffraction grating must be extremely small and precise. If the spacing between each of the grooves in the diffraction grating is not consistent, aberrations appear thereby reducing the resolution of the spectrometer. When the groove spacings are less than the 1.5 nm wavelengths employed in WDM systems, it is nearly impossible to precisely equally space the diffraction grooves without introducing some error.
FIG. 2 shows an example of a non-scanning spectrometer. As illustrated, the spectrometer includes a light signal source 10 coupled to a fiber 12 from which a light signal from light source 10 is projected. The spectrometer further includes a first lens 16 for collimating the light signal projected from an end 14 of fiber 12. The collimated light is projected onto a diffraction grating 18, which includes a large number of grooves. The grooves of diffraction grating 18 sample and reradiate the incident light signals. These temporal samples are separated by a fixed sampling interval. The spectrometer further includes a second lens 20 serving as a Fourier Transform lens that collects a portion of the scattered beam from diffraction grating 18 and relays the diffracted far-field pattern to a linear photodetector array 22. Linear photodetector array 22 supplies electrical signals representing the spatial relation and relative intensity levels of the resulting far-field diffraction pattern to a processing circuit 24. Processing circuit 24 includes a microprocessor 26 and a memory device 28, which may be a separate component of processing circuit 24 or may be integral with microprocessor 26. A display or printer may be connected to an output port 30 of processing circuit 24 to obtain a radiant of an estimate of the power spectrum of the light signal emitted from end 14 of fiber 12. Processing circuit 24 estimates the power spectrum of the incident light signals by analyzing the resulting far-field pattern that results from the diffraction of the incident light by diffraction grating 18 and which is sensed by linear photodetector array 22.
Depending upon the particular application of the spectrometer, conventional non-scanning spectrometers suffer from different deficiencies. When used as a PMD sensor, the resolution required is considerably smaller than the bandwidth of a modulated laser, which is typically about 20 GHz. Thus, an effective PMD sensor may require a resolution of 4 GHz. One would not expect to obtain the required spectral features of a PMD sensor using a non-scanning spectrometer since that would require using an echelle grating in a diffraction order in the hundred""sxe2x80x94a practical impossibility using currently available gratings.
When a non-scanning spectrometer is used as a WDM channel monitor, resolution is less of a problem since channel spacings are on the order of 50 GHz. However, when used as a channel monitor, non-scanning spectrometers require a high signal-to-noise ratio (SNR), which, in turn, has required a high precision grating. More specifically, any non-uniformity in the grooves in the grating raises the noise floor for the spectral estimate from the xe2x80x9cAiryxe2x80x9d floor, thereby resulting in deviations from what would otherwise be a perfect Airy function (also known as xe2x80x9cghostingxe2x80x9d). For a WDM channel monitor, a typical resolution of 40 or 60 dB may be required to properly identify the ASE noise floor, which otherwise would not be visible if the SNR of the spectrometer where smaller than that. Thus, it has generally been accepted that to improve the SNR, more precise, and hence much more expensive, gratings must be used.
Accordingly, it is an aspect of the present invention to provide a relatively inexpensive spectrometer having sufficiently high resolution for use as a PMD sensor or the like, that does not require the high precision diffraction grating of prior non-scanning spectrometers. It is another aspect of the invention to provide a spectrometer having a high resolution that is less susceptible to variation due to imprecise groove spacing of a diffracting member.
To achieve these and other aspects and advantages, a spectrometer of the present invention comprises an echelle array disposed in the path of a light signal so as to diffract the incident light signal. The light signal falls within a predetermined wavelength band that is centered about a central wavelength. The echelle array has a plurality of diffraction scattering sites periodically spaced apart by a distance of at least about five times the central wavelength. The spectrometer further includes a photodetector array positioned to receive a far-field diffraction pattern produced by the diffracted light from the echelle array and to output electrical signals representing the spatial pattern and relative intensity of the far-field diffraction pattern. Additionally, the spectrometer includes a processing circuit coupled to the photodetector array for processing the electrical signals to determine the power spectrum of the light signal.
It is another aspect of the present invention to provide an inexpensive WDM channel modulator having a sufficient SNR. Such a channel modulator may employ a less precise, and hence, inexpensive diffraction grating while increasing the SNR. To achieve this aspect and other advantages, the processor circuit may include a processor and a memory coupled to the processor. The processor calibrates by measuring far-field diffraction patterns and determining spatial impulse responses (SIRs) for light at a plurality of different known wavelengths, and stores the SIRs in a table in the memory. When the light signal is projected onto the echelle array, a far-field diffraction pattern for the light signal is obtained on the photodetector array, and the processor processes the output of the photodetector array to obtain an estimate of the power spectrum for the light signal. The processor provides the estimate of the power spectrum by deconvolving the SIRs obtained during calibration from the far-field diffraction pattern measured for the light signal.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.